Shiva hypothesis

• 1. Introduction

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  The Shiva hypothesis carries the name of a Hindu deity. It holds that global natural catastrophes, including extinction events, have struck Earth not randomly but at regular intervals. The mechanism proposed is cosmic in scope. According to the hypothesis, the Sun's periodic motion through the Milky Way galaxy triggers a chain of events. That chain ends with a heightened risk of large impacts on Earth. The Cretaceous-Paleogene extinction event is among the catastrophes the hypothesis invokes as supporting evidence. Two researchers first formalized this idea in the late 1970s, under a title that has long since been replaced.

• 2. Galactic Crossings

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  William Napier and Victor Clube published "A Theory of Terrestrial Catastrophism" in the journal Nature in 1979. Their argument centered on orbital geometry. The Solar System periodically crosses the plane of the Milky Way galaxy. Napier and Clube proposed that each such crossing generates gravitational disturbances powerful enough to reach the Oort cloud. The Oort cloud is the vast, distant reservoir of comets surrounding the outer Solar System. When those comets are disturbed, some are deflected inward toward the inner Solar System. This raises the probability of a large object striking Earth. The cycle, as the two researchers framed it, repeats roughly every 30 million years. Their original label for this framework would eventually give way to a different name.

• 3. Rampino and Renaming

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  Starting in 1984, Michael R. Rampino began publishing followup research on the hypothesis. A letter to the same journal that year, co-authored with Stothers, explicitly cited Napier and Clube's earlier work. Rampino's approach built directly on that earlier foundation.

  The hypothesis gained its current name in the 1990s. Rampino and Bruce Haggerty chose to rename the framework after Shiva, the Hindu god of destruction. Terrestrial catastrophism had acquired a new identity.

  Rampino's engagement with the hypothesis extended well into the following century. In 2020, he and colleagues published non-marine evidence that corroborated earlier marine evidence in support of the hypothesis. The two lines of data, one from the oceans and one from the land, gave the hypothesis a broader empirical base. Researchers working on independent explanations, however, had already proposed alternative mechanisms for the same pattern.

• 4. Rivals and Variants

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  Not every researcher who proposed a periodic-extinction mechanism focused on the galactic plane. One variant traces extinction cycles to the Solar System's passage through the Milky Way's higher-density spiral arms. A reanalysis using CO data found no correlation between that transit and mass extinctions.

  A brown dwarf named Nemesis forms the basis of a second alternative. This theory holds that an unseen companion to the Sun triggers extinctions every 26 million years. That interval differs slightly from the 30-million-year cycle at the center of the Shiva hypothesis. The Shiva hypothesis may have inspired the Nemesis idea. What both proposals share is the assumption that extinction events follow a predictable schedule.

• 5. A Problem with Periodicity

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  Extinction periodicity, the idea that mass die-offs recur on a predictable schedule, is the assumption critics have challenged most directly. The Shiva hypothesis requires that most or all extinction events share a common underlying cause. Evidence from the geological record, however, points to a variety of causes for different extinctions. Those varied causes are not the kind that a single, repeating astronomical cycle would consistently produce. A hypothesis built on cosmic rhythms must account for extinctions that arose through entirely different mechanisms at different times.

  Rampino and colleagues published non-marine corroborating evidence in 2020. The hypothesis that William Napier and Victor Clube set in motion in the late 1970s has outlasted the title they gave it.